Fiberboard and method of flameproofing the same



Feb. 24, 1959 L. R. DUNN ET AL FIBERBOARD AND METHD OF' FLAMEPROOFING THE SAME Filed May 31,1957

JM 4 mmmma we 5&0 4. 220.52230@ w 20m... m\ 20m...

ATTORNEY United States Patent O FIBERBOARD AND METHOD F FLAME- PROOFIN G THE SAME Lewis R. Dunn and Lewis W. Eckert, Lancaster Township, Lancaster County, Pa., assignors to Armstrong Cork Company, Lancaster, Pa., a corporation of Pennsylvania Application May 31, 1957, Serial No. 662,698 12 Claims. (Cl. 92-3) This invention relates to a iiberboard and method of ameprooiing the same. It is concerned more particularly with the flameproong of a cellulosic berboard such as one produced from wood fibers.

It has been proposed heretofore to flameproof fiberboard by applying to the surface thereof a coating consisting essentially of a mixture of borax and boric acid. The boric acid is primarily a glow inhibitor, and the borax serves chiefly to reduce flaming. `The mixture of borax and boric acid provides a flame-resistant surface. It has been found, however, that the borax and boric acid coated board, when in service where it is exposed to conditions of relatively high humidity, isV unsatisfactory, for the `borax tends to bloom to the surface, affecting the appearance of the resistance.

Attempts have been made to overcome 4this problem by eliminating the borax, but the nished product is inadequate in llame resistance. l

A cellulosic iberboard product can be produced according to this invention which is resistant to blooming at high humidities and which is adequately resistant to flaming to meet United States Department of Commerce commercial standard specification CS-42-49, class F.`

This is accomplished by treating the surface of the cellulosic iiberboard, preferably while still in a wet condition prior to drying, with a solution of boric acid so that the same penetrates into the body of the board below the surface. The product is then heated to react the boric acid with the `cellulosic fibers, preferably to a temperature of `about 240 F. The dry product is then coated with a composition including clay or similar finely divided inorganic'particles and a llame-resistant binder.

The` felted fibrous body or board may be formed essentially from wood fibers. A wood berboard prepared as disclosed in Scott Patent 2,642,359 forms a suitable base or body. The `iiberboard may be formed on a conventional berboard, making machine, such as an Oliver forming machine. `In this type of machine, a slurry of fibers suspended in water is deposited upon a rotating screen, and a continuous feltedlibrous sheet is formed from which a portion of the water is extracted. The formed sheet is then delivered to a press, such as a Downingtown press, and additional water is removed from the sheet, and the sheet is` consolidated to the desired thickness in the press.

The boric acid treatment may be effected by applying a solution of boric acid tothe formed sheet as delivered from theDowningtown press in a wet condition, prior to drying. The coating may be applied by'a roll coater, operating in advance of a pool of the boric acid solution which has been applied to the surface of the wet sheet.

`In order to` assist in the control Tof the penetration of the boric acid treating material into the wet board, a`

thickening agent such as sodium alginate` orthe like may be incorporatedinto thefboric acid solution.n

In the preparation-"of `a suitable treating material, the

product and reducing its llame` 2,875,044 Patented` Feb. 24,` 1959 2 following ingredients are combined, all parts being given by weight:

Boric acid 8.00 Sodium alginate (Keltex sold by Kelco Co.) 0.25 Water 91.75

This provides an aqueous acid solution, the effective or active component of which consists essentially of boric acid.

In the preparation of the treating solution, the water is heated to about 140 F., and the powdered sodium alginate is stirred in for about 5 to l5 minutes, until a uniform distribution of it in the water has been obtained. Thereafter, the powdered boric acid is added, and the mixture is stirred for about 5` minutes. The solution is maintained at a temperature of` about 140 F. in order to insure that the boric acid will remain in solution. If the quantity of boric acid is substantially greater than 8%, then the temperature of the solution should be maintained at a higher level in order to insure complete solution of the boric acid in the water. p

The boric acid solution in heated condition is delivered to the surface of the berboard sheet; and the speed of movement of the sheet through the machine,

quantity of boric acid solution deposited onto the liberboard and penetrating into the body thereof. Generally,

a quantity equivalent to about 4 to 20 grams of boricy acid per square foot of berboard surface (on a dry weight basis) is adequate for the purposes of the present invention. A quantity equivalent to about 10 to 1 5 graine per square foot is preferred. The application of the heated boric acid solution to the wet surface of the fiberboard (the boardas Vdelivered from the Downingtown press may have a surface temperaturel of about 100 F.) results in immediate interspersion of the boric acid treating solution and the Water contained in the upper portion of the board; a slight penetration of the boric acid solution into the body of the board occurs, the depth of penetration being controlled by the temperature of the solution, its viscosity, and other factors previously mentioned.

Due to this rapid interspersion of the boric acid solution into the top layer of the wet iiberboard,` thiswetend application method is preferred. The same nal results can, however, be obtained by applying the boric acid solution to a dried iiberboard sheet if provisions are made for adequate vpenetration of the solution, as for example through the use of wetting agents `with the moisture thus introduced into the dried iiberboard being subsequently removed by heat. t

The surface treated board is then delivered to a con-.

ventional drier, and water is removed from the board and the boric acid reacts with the bers in the area of penetration of the boric acid solution. In a typical drier installation, the first and middle zones may be heated to about 450 F. to 350 F. (from entrance to exit of each zone), and the exit zone may be similarly heated to about 370 F. to 260 F. It will be understood,

of course, that until such time as substantially all of theA water has been removed from the wet` board, the temperature of the board will not exceed labout 212 F.; and

in the inner areas, the board temperature will be much` lower. At completion of the drying operation, however,`

the board at the upper treated surface maybe elevated l to a temperature in the order of 220 FQto 250 F. The

board drying temperatures and the duration ofthe drying time will vary with the thickness of the board,the` board at the time `of` quantity of water contained in the delivery to the drier, the length of the drier zones, and

11C other variable factors. `The foregoing temperatures rare Water 1248.0 Trisodium pyrophosphate 3.0 Clay icoon Casein 75.0 Ammonia 6.0 Water 16.0 Pine oil 10.0

In the preparation of the coating composition, the trisodium pyrophosphate is dissolved in water and the clay is added. Mixing is continued until the clay is dispersed in the water. The trisodium pyrophosphate serves as a dispersing agent. Thereafter, the casein or other binder is added to the dispersion of clay and water, and mixing is continued. Following this, the ammonia, dissolved in the 16 parts of water, is added and mixing continued for a period of about two hours. The pine oil which serves as a wetting and leveling agent for lthe paint may be added at any convenient time during the process, and preferably during the mixing in of the ammonia treated casein solution.

Preferred practice .is to apply the primer portion of the top coating by means of .spray guns, followed by the application of heat and pressure to the surface by the use of heated ironing rolls. This tends to level the surface ofthe board by pressing down any projecting fibers which may extend above the surface. The heated lrolls also elevate the temperature of the surface of the board, removing moisture from the prime coating. The ironing rolls `may be heated by gas flames and have a surface temperature of about 900 F. They serve to elevate the temperature of the surface of the board to about 140 F. in a period of about one second.

The `finish portion of the top coat may be of the same composition as the prime coat, and it may be applied rby spray guns and then oven dried. Satisfactory results will be achieved if about 4 grams, dry weight, of the coating are applied per square foot of board surface in the prime coat` and about l grams, dry weight, per square foot in the finish portion. The total quantity of clay or `equivalent inorganic coating material applied over the boric acid treated layer will vary with the type of fiber inthe board, the smoothness `of the board texture, the size of the clay or other inorganic particles, and other variable factors. Preferably with-a wood iiberboard such as discussed above a total of about 142 to 25 grams per square foot on a dry basis will be used. For practical purposes, about 30 1grams per square foot is a reasonable upper limit, and as little as 4 grams will give some improvement with a smooth finished berboard.

An essentially complete covering of the inorganic particles over the boric acid treated layer of fibers appears to be desirable and is preferred. The exact action which occurs at the surface when the product isV exposed to a ame is not understood. It has been observed in the making of lire tests of the product, however, that there is a confining action which seems to limit the spread of the flame. Regardless of theory, the fire tests prove that it is Vessential to .have the inorganic coating of clay or the like disposed as a separate layer over the boric acid treated layer of fibers.

The nature of the chemical action involved between the boric acid and the wood libers i's not known. It has been found, however, that the combination of the boric acid treated layer of fibers and the clay layer disposed thereover provides a .flame-resistant `product which will 4 meet United States Department of Commerce commercial standard specification CS4249, effective November 15, 1949, and entitled Structural Fiber Insulating Board, fourth edition, page 3, class F. The same product with the boric acid coating alone or the same product with a clay coating alone will not meet the specifications. Also, if the clay is combined with the boric acid solution and the two are applied as a singletcoating, the product does not meet the applicable specifications. In these respects, a dried iiberboard sheet treated with a boric acid solution so as to secure adequate penetration behaves in the same fashion as the wet-end treated material.

While it is preferred to apply the clay top coat after the board has been treated with the boric acid solution, heated, and dried, as mentioned above, it is possible to apply the clay coating as a so-called wet-end coating, subsequent to the application of the boric acid solution and after penetration of the boric acid solution below the surface of the board.

The following composition may be applied as a wetend coating, all parts being given by weight:

Total solids about 50%.

n preparation of the composition, the starch and water of Part B are mixed and heated to about F. to gel the starch. This gel is added to the clay vslurryof Part A.

The mixture may be applied at 4room temperature to the wet board, after application of the boric acid solution and penetration of .it below the surface of the board. It is preferred to use about `12 to 25 grams kof this coating on a dry basis, over l0 to 15 grams of boric acid, also on a dry basis. A decorative coating may be applied and ironed onto the surface at the dry end, if desired, but is not necessary.

With a board 1% thick fand made of `fiber such :as disclosed in .the Scott patent referred to, the penetration of the boric acid 'may be in the Aorder of 1/6"",.and satisfactory results will be achieved. There is no critical limit to :the depth'of penetration of the lboric acid treating solution, although for most purposes the thickness of the boric acid layer need not exceed about 1/16 to Vs. As `mentioned previously, the `dry weight of boric acid per square foot of board is Vpreferably l0 to 15 grams, andthe dry weight of clay or the like `in the top coat is preferably l2 to 125 grams with a wood berboard made by l.the process `of the Scott patent.

Where a thickening :agent is used with the boric acid solution, the sodium alginate is preferred; but other thickening agents, such as carboxy methylcellulose, starch, and Ibentonite, may be substituted therefor las iswellknown in the coating art. Generally, only a very small quantity of thickening agent will be used, `in the :order of 0.25% to 1.0% of the total weight of the `treating solution.

In place of casein as the binder for the top coating composition, other binders may be1used,such as antimony trioxide and chlorinated paraflin, polyvinyl acetate, and the like. They should, of course, be lflame-resistant.

Calcium carbonate, atomite, '.whiting, lithopone, ttani. um dioxide, and `the ylike and vmixtures ythereof :may'be substituted :in whole or lin part for `the clay. 'f

While Athe preferred base-or body isa wood fiberboard,

it is obvious Vthat .other similar products may be sub--A stituted therefor, .such as berboards made 'f-romfsugar cane fibers, ground hard or softwood, and other cellulosic materials.

i A drawing is attached in which:

Figure 1 is a diagrammatic view which illustrates one method of carrying out the invention; t

i Figure 2 is a diagrammatic view'whichu illustrates another method of carrying out the invention; and

Figure 3 is `a diagrammatic view of a piece `of fiberboard embodying the invention. The various elements of the product have been labeled in this view.

In the embodiment of the method shown in Figure 1, the wood fiberboard 2 is supported on rolls 3 and is delivered lfrom a Downingtown press to a station 4 where a roll coater 5 applies the boric acid solution to the surface 6 of the 'board from a pool 7 disposed to the rear of the roll coater 5 in the direction of travel of the board as indicated by the arrow shown in the drawing.

As mentioned previously, preferred practice is to deliverthe board thus treated with the boric acid solution to a drier for heating of the boric acid solution and drying of the board. (A break in the iberboard 2 has been indicated in the drawing to diagrammatically. illustrate this.) Thereafter, shown in the drawing where the prime portion of the top coat is applied from a spray gun S to the surface and at` the same time a coating is applied from a roll coater 9 to the bottom surface of the board in order to minimize the possibility of warpage. The prime coating applied to the surface is engaged by a heated ironing roll 10 which is traveling in the direction indicated by the arrow in the drawing, the same direction as the direction of movement of the board through the machine. The roll 10 is heated `by a gas burner 11, and its surface temperature may be elevated to a high range, as noted previously. A corresponding ironing system may be applied to the under s-urface of the board. While a single ironing operation may be satisfactory, it is preferred to have two sets of ironing rolls, the second set of rolls being indicated at 12 in the drawing. They are heated by gas flame arrangements similar to the arrangement 11, and here too there is a lower ironing roll to compensate for the action of the upper roll and minimize warpage. t

Spray guns 13 and 14 may be used to apply the finish portion of the top coat to the upper surface of the board. Since the lower surface has been set by the ironing operation, it is not necessary to apply any treating solution to the lower surface although a coating may be applied if desired. Thereafter, the material is delivered to a drier for final drying of the top coating.

In the embodiment of the method shown in Figure 2, the boric acid is applied to the surface of the berboard 2 in the same manner as in Figure l; but in this embodiment, the top coat of clay is applied as a wet-end coating at station 15 where a roller coater 16 applies the coating composition previously mentioned to the surface 17 of the board from a pool 18 disposed to the rear of the roll coater 16 after the lboric acid solution has been applied and has penetrated into the body of the wet board. A break has been indicated in the board to show an interval between the two roll coaters. With the top coating applied at the wet end to a wood fiberboard, preferred practice is to deposit over the boric acid treated surface about 20 to 25 grams per square foot of top coating material, on a dry basis. After application of the boric acid and the top coating, the board may be delivered to a conventional drier moved and the boric acid will be reacted with the cellulosic fibers. The drying temperatures may be those previously mentioned in connection with the drying of a fiberboard having a boric acid treating solution applied at the wet end.

The boric acid solution prevents discoloration of the board surface and thus permits the use of higher drying and reaction temperatures than would otherwise be possible. This is more fully disclosed in Eckert application,

the top coating may be applied as` where water will be re-` 6 Serial No. 519,256, led June 30,` 1955, and entitled Method of Drying Water-Laid Fibrous Materials. This application is a continuation-impart of our copending application Serial No. 508,742, filed May 16, 19.55, now abandoned, and entitled Fiberboard and Method of Flameproofing the Same.

We claim:

1. In a process of rendering fiberboard and the like name-resistant, the steps comprising forming a multiple layer system on the uppersurface of the berboard, including a layer made up of cellulosic fibers reacted with -boric acid and a separate layer disposed thereover and made up of finely divided inorganic particles and a flameresistant binder, by applying to the surface of a formed body of felted cellulosic fibrous material an acid solution the active component of which consists essentially of boric acid, which solution penetrates only to a limited extent into the formed body below the surface thereof and said boric acid thus is concentrated in a surface layer adjacent to the upper surface of the formed body, applying to the upper surface of the formed body over the boric acid coated fibers prior to drying a separate iiameresistant layer of finely divided inorganic particles and a dame-resistant binder, and at some stage in the process heating the boric acidcoated fibers to an elevated temperature at which said =boric acid is reacted with said celluosic fibers. t i i 2. In a process of rendering fiberboa'rd and the like dame-resistant, the steps of claim 1 in which the boric acid concentrated in said surface layer constitutes 4-20 grams per square foot on a dry weight basis.

3. In a process of rendering tiberboard and the like flame-resistant, the steps of claim 2 in which the finely divided inorganic particles of said separate flame-resistant layer constitute 4-30 grams per square foot on a dry weight basis.

4. In a process of rendering fiberboard and the like dame-resistant, the steps comprising applying to the surface of a formed body of felted cellulosic fibrous material an acid solution which consists essentially of boric acid and solvent therefor, which solution penetrates only to a limited extent into the formed body below the surface thereof and said boric acid thus is concentrated in a surface layer adjacent to the upper surface of the formed body, heating said surface layer to a temperature at which reaction between said boric acid and said cellulosic fibers occurs, and forming over said surface layer a separate dame-resistant layer which consists essentially of finely divided inorganic particles and a. name-resistant binder.

5. In a process of rendering iiberboard and the like flame-resistant, the steps comprising applying to the surface of a formed body of felted cellulosic fibrous material in wet condition an acid solution which consists essentially of boric acid and water, which solution penetrates only to a limited extent into the formed body below the surface thereof and said boric .acid thus is concentrated in a surface layer adjacent to the upper surface of the formed body, heating said surface layer t0 a temperature above about 220 F. to react said boric acid with said cellulosic fibers in said surface layer, and forming over said surface layer a separate flame-resistant layer which consists essentially of finely divided inorganic particles and a flame-resistant binder.

6. In a process of fabricating a name-resistant fibe'rboard, the steps comprising forming a slurry of felted cellulosic fibers into a sheet, removing a portion only of the water from the sheet, applying to the wet surface of the sheet an acid solution consisting essentially of boric acid and solvent therefor, which solution penetrates only to a limited extent into the formed body below the surface thereof and said boric acid thus is concentrated in a surface layer adjacent to the upper surface of the formed body in the amount of 4-20 grams per square foot, heating the formed body to a temperature of 220 F. to 250 F. Vto react said boric acid with saidcellulosic fibers in said surface layer, depositing as a separate layer ove-r the surface layer a priming composition which consists essentially of nely divided inorganic particles and a flame-resistant binder therefor, heat and pressure ironing said surface'coated with said priming `composition to dispose within the body of the iberboard projecting bers which extend therefrom, and depositing a second coating of essentially the same composition as the priming composition over the ironed surface.

7. In a process of rendering iiberboard and the like dame-resistant, the steps comprising applying to the surface of a formed body of felted cellulosic fibrous material in wet condition an acid solution the active component of which consists essentially of boric acid, which solution penetrates only to a limited extent into the formed body below the surface thereof and said boric acid thus is concentrated in a surface layer adjacent t0 the upper surface of the formed body, applying to the upper surface of the formed body over the boric acid coated fibers in Wet condition a separate flame-resistant layer of finely divided inorganic particles and a flameresistant binder, and then heating said berboard product to an elevated temperature at which said berboard product is dried and said boric acid isvreacted with said cellulosic fibers. Y

8. A flame-resistant fiberboard comprising .a body of cellulosic material carrying on one surface thereof and disposed within a limited area only of the body below;

imposed thereon and extending thereabove as a separatecoat a layer of linely divided inorganic particles and a flame-resistant binder.

9. A flame-resistant iberboard in accordance with claim 8 in which the finely divided particles comprise clay.

10. A flame-resistant berboard in accordance with claim 8 in which a casein binder is used in said separate coat.

l1. A flame-resistant tiberboard in accordance with claim 8 in which the boric acid reacted fibers extend 1/8" or less from the surface of the board.

12. A flame-resistant tiberboard comprising a body of cellulosic material carrying on one surface thereof and disposed within a limited area only of the body below said surface the dried residue of the heat reaction product of said cellulosic bers an acid solution consisting essentially of boric acid and Water, and superimposed thereon and extending thereabove as a separate coat a layer consisting essentially of finely divided inorganic 25 particles and a name-resistant binder therefor. 

1. IN A PROCESS OF RENDERING FIBERBOARD AND THE LIKE FLAME-RESISTANT, THE STEPS COMPRISING FORMING A MULTIPLE LAYER SYSTEM ON THE UPPER SURFACE OF THE FIBERBOARD, INCLUDING A LAYER MADE UP OF CELLULOSIC FIBERS REACTED WITH BORIC ACID AND A SEPARATE LAYER DISPOSED THEREOVER AND MADE UP OF FINELY DIVIDED INORGANIC PARTICLES AND A FLAMERESISTANT BINDER, BY APPLYING TO THE SURFACE OF A FORMED BODY OF FELTED CELLULOSIC FIBROUS MATERIAL AN ACID SOLUTION THE ACTIVE COMPONENT OF WHICH CONSISTS ESSENTIALLY OF BORIC ACID, WHICH SOLUTION PENETRATES ONLY TO A LIMITED. EXTENT INTO THE FORMED BODY BELOW THE SURFACE THEREOF AND SAID BORIC ACID THUS IS CONCENTRATE IN A SURFACE LAYER ADJACENT TO THE UPPER SURFACE OF THE FORMED BODY, APPLYING TO THE UPPER SURFACE OF THE FORMED BODY OVER THE BORIC ACID COATED FIBERS PRIOR TO DRYING A SEPARATE FLAMERESISTANT LAYER OF FINELY DIVIDED INORGARNIC PARTICLES AND A FLAME-RESISTANT BINDER, AND AT SOME STAGE IN THE PROCESS HEATING THE BORIC ACID COATED FIBERS TO AN ELEVATED TEMPERATURE AT WHICH SAID BORIC ACID IS REACTED WITH SAID CELLUOSIC FIBERS. 